I am also an ex-runner. You will want a road bike for your uses. Unless it is a total piece of junk, the brand does not matter. So don't buy a bike from a department store, but any reputable brand (and even some that aren't very well-known) are just fine. The truth is, most of them are made in the same few factories anyway, then labeled with their brand. Fit is terribly, terribly important though. If the bike doesn't fit, you have nothing. And if you buy used, make sure you take it in to get a thorough tune up. If you have a bicycle collective where you live, they will restore older bikes, tune them, and sell them, which is ideal.

There might be an equivalent of C25K, but IMO, you don't need it. Once you get a bike that fits and functions, just go ride a lot. As one who has finished quite a few marathons and century rides, the century rides are honestly a lot easier.